A communication session, such as a voice or video call, can assume different session states. For example, in the course of establishing the communication session, two or more different session states are subsequently assumed by the communication session. For processing the communication session and for interpreting control signals, network components and terminals involved in the communication session have to agree on the current session state. However, when the communication session is subjected to a transfer, an interruption of a communication link between a network component and a terminal can bring the session state at the network component and the session state at the terminal out of synchronization.
By way of example, a network operator providing service coverage according to Universal Mobile Telecommunication System (UMTS) may want to move voice services to Voice over Internet Protocol (VoIP) using an Internet Protocol Multimedia Subsystem (IMS) in a Core Network (CN) in conjunction with a Radio Access Technology (RAT) according to Long Term Evolution (LTE). Since UMTS provides voice services in the circuit-switched domain, a call session initiated in the packet-switched domain according to LTE has to be transferred from the RAT in the packet-switched domain to another RAT in the circuit-switched domain. However, a message relevant for the session state can be lost in the context of the transfer, so that a User Equipment (UE) and a network component assume different session states.
As an example for the transfer, Single Radio Voice Call Continuity (SRVCC) according to standard document 3GPP TS 23.216 (Release 11, Version 11.6.0) specifies a technique for service continuation of IMS voice calls. The IMS voice call can, for example, originate in the packet-switched domain using a conversational VoIP bearer according to the Quality of Service (QoS) Class Identifier “QCI1”. The QCI1 VoIP bearer is merely one example for an Evolved Packet System (EPS) bearer. In UMTS and GPRS, data sessions are established differently using the Packet Data Protocol (PDP) context procedure. SRVCC allows transferring a voice call from the packet-switched domain of LTE using VoIP and IMS to the circuit-switched domain of UMTS. SRVCC thus provides a fallback position when introducing VoIP services according to the LTE standard.
Standard documents 3GPP TS 23.237 (Release 12, Version 12.1.0) and 3GPP TS 24.237 (Release 11, Version 11.4.0) specify the SRVCC transfer for video and voice calls in an Alerting Call State. The Alerting Call State of a session may be defined as the state of a party engaged in a session that is in an early dialog state according to Sect. 12.1 of RFC 3261 (Version of June 2002). Alternatively or in addition, the Alerting Call State of the call may be defined as the state of a session with a speech media component after exchanging an initial “SIP INVITE” request that establishes the dialog and a “SIP 180” Ringing response that indicates the alerting, and before exchanging a SIP final response for the initial “SIP INVITE” request that establishes the session according to Clause 12.1 of standard document 3GPP TS 24.237.
To tackle a possible incompatibility between the session state of the calling UE and the session state of a Mobile Switching Center (MSC) server due to a radio interruption during the SRVCC transfer, the standard document 3GPP TS 24.237 recommends that the MSC server verify the call state using the “STATUS ENQUIRY” procedure of standard document 3GPP TS 24.008 (Release 12, Version 12.0.0). If the call states are incompatible, the transferred session is released according to standard document 3GPP TS 24.237, Clause 12.6.3, Note 3.
Document WO 2011/139045 A2 teaches a technique for synchronizing different call states at the remote other end of the dialog. Here, an incompatibility is caused by the called UE answering the call just before the called UE is handed over from the packet-switched mode to the circuit-switched mode according to SRVCC. No solution is provided for synchronizing the calling UE and the telecommunications network from which the call is originating.
Thus, incompatible session states can arise in the prior art and lead to an unexpected termination of the session.